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OBESITY is “socially contagious”, with people who have an overweight friend facing a dramatically increased risk of becoming similarly fat.

A study of more than 12,000 people found that if someone was obese, the chances their friend would also become obese increased by 57 per cent.

Their sibling’s risk of obesity would also increase 40 per cent and their spouse by 37 per cent.

The researchers, from Harvard Medical School and the University of California, said this suggested obesity was “socially contagious”, spreading from person to person in a social network. But they said the greatest effect was not among people sharing the same genes or household, but among friends.

Professor Nicholas Christakis, co-author of the study, said: “It’s not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct causal relationship.” He said the connection was not as simple as saying that friends adopted each other’s lifestyles. “It’s more subtle than that,” he said.

“What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size.

“People seem to think that it is okay to be bigger since those around them are bigger and this sensibility spreads.”

The research, published in the New England Journal of Medicine, covers a period of 32 years. James Fowler, a co- author of the study, said: “Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much.

“Social effects, I think, are stronger than people realised.

When we help one person lose weight, we’re helping many.” He added: “We’ve not only shown obesity is contagious, but thinness is contagious too.”